1. Field of the Invention
The present invention relates to active pixel sensors. More specifically, the present invention relates to reducing readout delay in an active pixel sensor.
2. Description of Related Art
Many systems include imaging devices to sense and capture optical images that can be electronically converted to a digital representation of the image. Image sensors include an array of photo-sensitive devices such as photo diodes or photo-transistors fabricated on, for example, a complementary metal oxide semiconductor (CMOS) device. Each photo-sensitive device is sensitive to light in such a way that it can create an electrical charge that is proportional to the intensity of light striking the photo-sensitive device. The overall image captured by an image sensor includes many pixels arranged in an array such that each pixel detects the light intensity at the location of that pixel.
Imaging devices formed according to a conventional CMOS process are known as a CMOS imager and may be configured to include active pixel sensors (APS). An active pixel sensor (APS) consists of an integrated circuit containing an array of pixels, each containing a photo detector (e.g., photo diode or other similar device) as well as other transistors for resetting and gating the stored charge on the photo detectors. In a conventional CMOS imager, each pixel cell in an array of pixels operates to convert light intensity to electrical charge, accumulate the electrical charge in proportion to the light intensity, and transfer the accumulated charge to an amplifier. In many CMOS imagers, a pixel may be reset to a specific reference voltage level prior to, or after, acquiring the image.
The circuitry for retrieving or obtaining a reading of the accumulated charge corresponding to the light intensity is conventionally known as “readout circuitry.” Various implementations of readout circuitry are constrained by slow signal settling or transitioning of the output signal in response to reading of the stored charge on the photo diode. Furthermore, while the photo detectors are just a portion of the actively switching circuits in a system, electrical transients from other switching circuitry may inadvertently introduced transient variations to the power supply voltage range resulting in a skewed output signal.
Therefore, there is a need for circuits, devices, methods and systems for reducing signal settling time and reducing supply line transients in an APS array.